1. Field of the Invention
The present invention relates generally to free rotor gyroscopes and, more particularly, to double gimbal flexure suspensions for dynamically tuned, free rotor gyroscopes.
2. Description of the Prior Art
Free rotor gyroscopes such as Applicant's Assignee's U.S. Pat. No. 3,529,477, and dynamically tuned, free rotor gyroscopes of the type disclosed, for example, in an article entitled "The Dynamically Tuned Free Rotor Gyro" by E. W. Howe and P. H. Savet, appearing at pages 67-72 of the June 1964 issue of Control Engineering and in U.S. Pat. Nos. 3,678,764 and 3,943,778, are well known in the art. Further, double gimbal flexure suspensions for such gyros are disclosed in, for example, U.S. Pat. Nos. 3,832,906; 3,856,366; 3,943,778; 4,062,600 and 4,100,813.
In the flexures known to the prior art, as exemplified in the latter group of the above patents, and as typically shown in FIG. 1A, each of the four flexure blades of each of the two gimbals is formed by machining two holes very closely together such as to leave a very thin, generally tangential wall section "t"--on the order of one-to-two thousandths of an inch between the holes. This design configuration has several design deficiencies as will be discussed in the following paragraphs.
The presence of material defects in the flexures is a potential problem with the prior art mentioned. These flexure assemblies are fabricated from one or two relatively large pieces of material having the required end-product physical characteristics. Although relatively clean alloys can be achieved by current vacuum melt techniques, some lack of homogeneity is always possible. A flaw or material defect in any one of the thin flexures can lead to the loss of the entire assembly during the manufacturing process and/or failure of the instrument in service.
The potential for flexure damage during the manufacturing and calibration process is high for the prior art designs. Referring to FIG. 1A, the holes "D" must be drilled, bored, reamed, and polished to extremely precise dimensions. Since the wall thickness "t" of the flexure is of the order of one-to-two thousandths of an inch, these operations must be conducted with great care to prevent permanent deformation or other damage to the flexures. If EDM (Electrical Discharge Machining, a well understood technique) is employed, the basic holes can be formed precisely with no stressing of the flexures, but additional difficulties arise; for example, a thin layer of material known as the recast layer is formed in which there are surface cracks and metalurgically altered material. If this recast layer (typically one thousandth of an inch thick) is not removed, the flexure will have a drastically reduced fatigue life. Therefore, the recast layer must be removed by polishing, etching or otherwise, further complicating the manufacture of the flexure assemblies and increasing their costs. This is also true for flexures fabricated by EDM techniques in configurations other than the twin hole technique.
The precision machining required for a good quality flexure assembly of the prior art is difficult, time-consuming, and costly. The flexure spring rates required for miniature, low-speed, dynamically-tuned gyroscopes, are typically 0.004 inch pound/radian per flexure, or less. Since miniaturization limits the basic hole diameter "D" to approximately 0.047 inches, the effective length "1" of the flexure is approximately 0.010 inches. Consequently, the flexure thickness "t" must be extremely thin, usually less than one-thousandth inch in order to achieve the required spring rate. Furthermore, the spring rate of each of the flexures of the complete gimbal flexure assembly, involving at least 8 flexures, should be very closely matched and the angular spacing of the flexure axes must be precise in order to minimize torque rectification due to vibration at 2.times.N frequencies. Typically, spring rate matching requires additional material removal from the flexure blades as a calibration step.
The complexity of the inertial tuning and center-of-gravity gimbal adjustments required on the prior art designs contributes to high costs. In the cited prior art, double gimbal flexures, the inertias and centers-of-gravity of each of the gimbals is adjusted to achieve cancellation of spring rates at the desired operating speed N and cancellation of rectification torques due to linear and/or angular vibrations at 2.times.N frequencies. For example, each of the gimbals will produce torque rectification due to 2.times.N vibration but the rectification torque vectors will be equal in magnitude but opposite in direction and thus cancel each other provided that the spring rates, flexure axes spacing, inertias, and centers-of-gravity of the gimbals are carefully matched for this characteristic. In the prior art, the inertia and center-of-gravity adjustments are accomplished by a plurality of adjustable balance weights on both of the gimbals, these weights comprising screws axially adjustable in a plurality of tapped holes around the periphery of the gimbal. This balance weight configuration requires the precision location, drilling and tapping of the gimbal holes (as many as eight) thereby increasing manufacturing costs and requiring complex and tedious adjustment of each screw.
Prior art flexure designs do not make provisions for balancing the gyro to minimize torque rectification due to synchronous vibration along the spin axes at 1.times.N frequencies. This error torque originates due to the radial location of the center-of-gravity of the gyro rotor not being coincident with the center of support provided by the flexure suspension system. If not calibrated, errors of this type can be as high as 100.degree./hr per g of vibration amplitude. Proper calibration is achieved by vibrating the assembly at 1.times.N frequencies and balancing the rotor to minimize this effect. Since this will lead to an over-all rotational dynamic imbalance of the gyro, provisions must be made to correct this imbalance by a "shaft-fixed" balancing system.
Furthermore, in the prior art flexures, the gimbal associated with the spin shaft are machined from blanks separate from the shaft and thereafter secured to the shaft. This technique leads to misalignment of the flexure axes relative to the spin axis of the gyro, which degrades the over-all performance of the gyro--especially in the presence of dynamic inputs.